This invention concerns subsea completions and more particularly relates to a completion arrangement that may be used to provide a large bore, high pressure, conventional (i.e. non-horizontal), concentric Christmas tree and tubing hanger system.
For deep water developments it is now considered that conventional trees have advantages over horizontal trees. The horizontal tree concept has proven to be less advantageous than originally forecast in terms of installation times and design complexity. A demand has therefore arisen for a large bore conventional tree.
As the size of the production bore in a conventional Christmas tree and tubing hanger arrangement is increased, a large offset is often provided between the wellhead centerline and the fluid conducting bores at the tubing hanger/tree interface, primarily to avoid excessive enlargement of the tree block. In the case of parallel bore tubing hangers, this offset can arise in each of the production and tubing annulus bores. For concentric tubing hangers, there need be no offset in the production bore, but the tubing annulus offset is correspondingly larger. A very large offset in either the production bore or the tubing annulus bore will prevent wireline access.
During installation of parallel bore tubing hangers it is necessary to set wireline plugs in both bores. This requirement therefore restricts the permissible bore offsets. Tubing hangers are available which have a hydraulically operated annulus isolation valve rather than a plug. However it has usually been the practice to provide wireline access to this valve, for emergency operation in case of hydraulic actuator failure. Provision of such access and the consequent need to avoid dog legs at the tubing hanger/tree interface makes the tubing hanger and tree design relatively bulky and incapable of accommodating large numbers of downhole service lines.
Providing full wireline accessibility in a large bore conventional completion therefore leads to a large and heavy Christmas tree and tubing hanger installation. The upper weight limit for the lifting gear used to transfer equipment between supply and installation vessels is approximately 35 tonnes. This limit is reached for a conventional Christmas tree for use with 5xc2xd inch (140 mm) tubing. We have realized that improved space utilization and various consequential design improvements are possible, both in the tubing hanger and in the tree, if wireline accessibility for the annulus isolation valve is abandoned. By this means the tubing size can be increased to seven inches (178 mm) or more while maintaining the tree weight within the 35 tonne limit and the tree dimensions likewise within acceptable limits.
Against this background, in accordance with a first aspect of the invention, we provide a tubing hanger forming a substantially centrally located production bore and a radially offset tubing annulus passage selectively closeable by a remotely operable valve; the tubing hanger upper end in use co-operating with a Christmas tree to define a void in which couplers for service lines running from the tree through the tubing hanger are accommodated; the valve communicating with the void whereby the couplers are bathed in fluid flowing to or from the annulus passage.
For large diameter production bores, this arrangement makes effective use of the space available across the horizontal section of the tubing hanger, with plenty of room around the periphery of the production bore for accommodation of service lines. For example, a hanger for seven inch (178 mm) tubing according to the present invention may accommodate up to 8 service lines; whereas the maximum number of service lines that can be accommodated in a comparable prior art parallel bore tubing hanger is 4. Currently tubing hangers for horizontal trees can only accommodate a maximum of 7 service lines. We have found that it is unnecessary to include sealing arrangements for isolating this fluid from the couplers, contrary to established practice with some prior hanger designs. The present invention may therefore provide a particularly simple and compact tubing hanger annulus passage to Christmas tree seal, capable of accommodating a relatively large number of service line couplers.
For most efficient space utilization, the valve is preferably located at the upper end of a tubing annulus passage in the tubing hanger. The valve is preferably pressure balanced, thereby requiring actuators of relatively small size and the same time being intrinsically reliable. Fluid communication between the void and the Christmas tree is preferably by means of one or more drillings extending from the void into the Christmas tree.
In a further aspect, the invention also provides means for orienting a tubing hanger and a tubing hanger running tool. Prior methods of aligning a tubing hanger and a tubing hanger running tool have included the use of an orientation joint above the tubing hanger running tool. This usually has an orientation helix and a keyway or the like, which interact with a pin or key projecting into the BOP interior. Alternatively the pin, helix and keyway may be provided between the tubing hanger running tool and the BOP. Both these arrangements involve complex manufacturing and care in use. Unless detailed records of the BOP used during the original installation of the tubing hanger are still available, there may be difficulties in setting up the orientation equipment correctly. A further orientation method involves actively rotating the completion riser at the surface to bring a spring loaded key on the running tool into alignment with a corresponding keyway in the tubing hanger. This method is impractical for deep water applications.
This further aspect of the invention provides a tubing hanger forming a substantially centrally located production bore, and a tubing hanger running tool; the running tool including a recess longitudinally engageable with a projection extending from the tubing hanger production bore, whereby the running tool may be orientated for engagement with the tubing hanger. High tolerance alignment of the running tool to the tubing hanger may thus be achieved directly and independently of any BOP; such alignment being passive, i.e. arising automatically as the running tool is landed on the tubing hanger, and effective even in deep water. The tool may also include an orientation helix to guide the projection into the recess. As it is associated with the tool, this helix does not restrict the tubing hanger production bore or require additional space within the tubing hanger. The tubing hanger production bore may also include a recess or projection for longitudinal engagement with a complementary projection or recess on a Christmas tree to provide alignment between the tree and tubing hanger.
Abandonment of wireline access to the tubing hanger annulus passage also allows a simplified, more compact and hence lighter Christmas tree to be used. Accordingly, in another aspect, the invention provides a Christmas tree having a body within which is formed a production flow bore having a lower end for connection to a tubing hanger production bore, and a tubing annulus conduit having a lower end for connection to a tubing hanger tubing annulus passage; the production flow bore and the tubing annulus conduit being interconnected by a crossover conduit formed within the tree body; the tubing annulus conduit including a deviation within the tree body, allowing room for a valve in the crossover conduit. The need for an external, separately formed, crossover conduit is thus avoided. Preferably the tree production flow bore has an upper end at the top of the tree body and is sufficiently aligned with the tubing hanger production bore to allow wireline access to the tubing hanger production bore through the tree production flow bore upper end. More preferably, the tree production flow bore is coaxial with the tubing hanger production bore which in turn is substantially centrally located within the tubing hanger.
As the production bore of a conventional tree increases in diameter, the Christmas tree height and weight also increase, partly for the reasons discussed above, concerning wireline accessibility, and partly due to the need to use larger valves. In accordance with a yet further aspect of the invention, a compact, relatively lightweight Christmas tree forms a production flow bore having a lower end for connection to a tubing hanger production bore, and a tubing annulus conduit having a lower end for connection to a tubing hanger tubing annulus passage; at least two removable plugs being provided in series in the tree production flow bore to act as pressure barriers. In conventional trees, at least one of these barriers, and more usually both, are provided by means of gate valves having large and heavy actuators. Substitution of the gate valves by plugs therefore saves considerable bulk and weight. The plugs are preferably wireline installed crown plugs.
The various aspects and preferred features of the invention are described below with reference to illustrative embodiments shown in the drawings.